Mouse cursors are mostly handled in hardware. GPUs composit the cursor during scanout, so all the OS has to do is calculate the new coordinates of the cursor and tell the GPU about them.
> Mouse cursors are mostly handled in hardware. GPUs composit the cursor during scanout, so all the OS has to do is calculate the new coordinates of the cursor and tell the GPU about them.
Is this true on modern Linux DEs (e.g. on KDE Plasma)?
I don't know about other platforms, but in Windows, it had been true >20 years ago, before the desktop was even composited. Some fullscreen games also relied on this functionality, and sometimes you had an option in game settings to switch between hardware-accelerated cursor and manually drawn one (because the former could be buggy with some video drivers).
This is fairly basic functionality. Windows and wlroots-based compositors have it at least. I'm reasonably certain that other major compositors have it too.
If you want to know my epistemic status: I know about windows based on observable surface behavior and bugs related to accelerated cursors, but I haven't looked at the source. I know about wlroots because I saw the pull requests related to that and a flag to disable it in sway. The last statement regarding other platforms was an educated guess based on gfx card history: accelerated overlays are an ancient feature present in a lot of hardware, not some newfangled niche feature.
You can actually test this pretty easily by monitoring GPU usage. When nothing on the screen changes, GPU usage should be pretty much zero, even at the lowest P-state (because the compositor isn't rendering new frames unless something changed). You can move the cursor around and GPU usage will stay at the same level, but do something that actually requires new frames to be rendered, like for example dragging a selection rectangle on the desktop, and you'll see GPU usage go up a bit. (And if you do the same in applications using GPU-accelerated frameworks you'll often see the GPU rev up a few P-states, for example scrolling in the Steam library list is enough to cause a transition for P2 for me, likewise many websites kick it up to P2 when you move the mouse or scroll). Another way to see how the cursor is not using the regular rendering pipeline is to move a window around. You'll invariably see that the window lags behind the cursor by 1-3 frames. Other ways where this becomes noticeable is when the graphics stack breaks down, but the cursor still works.
Just an FYI-- I tried to get ChatGPT to generate a question as condescending as what you just responded to. It now generally refuses to write anything impolite.
The most I could get was to ask it to take an avuncular tone, at which point it did ask you to "spill the beans, kiddo" about these compositors. :)
